1. Field of the Invention
The present invention is related to a Fourier-domain optical coherence tomography imager and its operation.
2. Discussion of Related Art
Optical coherence tomography (OCT), developed in 1991 by MIT, has become an important medical imaging methodology, especially in ophthalmologic applications. The OCT technique is based on combining optical interferometer spectra from a longitudinal scan (also referred to as an axial scan, A-scan, or Z-scan) with a lateral scan (also referred to as an XY-scan). The XY scan can be in any lateral scanning pattern, including lines, circles, raster type scans, or any other pattern. The OCT image is constructed from optical interference signals while the OCT imager is performing A-scans and XY-scans. In most OCT devices, the A-scans are taken at a series of XY coordinates. In other words, an A-scan is taken at each point in an XY-scan to make a complete OCT image. As a result, the imaging speed is determined primarily by the A-scan scanning speed.
FIG. 1 illustrates an OCT imager 100 that can utilize the OCT technique. As shown in FIG. 1, a light source 101 provides light through a coupler 102 to a sample arm 103 and a reference arm 104. Reference arm 104 provides a known length and reflects light back into coupler 102. Sample arm 103 provides light to sample 106, which can be any object-of-interest, including an eyeball. The reflected light from sample arm 103 and the reflected light from reference arm 104 are combined in coupler 102 and the combined signal is coupled into detector 105.
FIG. 2 illustrates various scan types that can be performed in sample 106. Sample arm 103 can include optics for scanning light laterally across sample 106 to provide an XY-scan. The length of reference arm 104 may be mechanically varied to provide an A-scan or the A-scan can be the result of diffraction techniques utilized in detector 105. The image of the eye by OCT is measured by the interference between light from the sample arm 103 and reference arm 104 at detector 105.
OCT techniques can be divided into either time-domain OCT or Fourier-domain (or spectrum-domain) OCT. In time-domain OCT, the A-scan is usually provided by a mechanical scanning device in reference arm 104. Because of limitations in the mechanical scan, the A-scan speed in time-domain OCT is typically less than 5 kHz. Further increasing the scan speed may result in a poor signal-to-noise ratio because of the unmet requirements of wider electronic signal bandwidths. In Fourier-domain OCT, however, the A-scan is usually provided by a spectrometer in detector 105. The Fourier-domain OCT spectrometer typically includes a line-scan camera coupled to a diffraction grating. The line-scan camera, therefore, receives an optical interference signal as a function of wavelength. The OCT image can then be constructed after performing a Fourier transform on signals received at the line-scan camera. Because the line-scan camera can have a very high scan rate (typically >25 kHz), Fourier-domain OCT can provide a much higher imaging speed than time-domain OCT. Furthermore, Fourier-domain OCT can also provide a higher signal-to-noise ratio than time-domain OCT at the same A-scan rate, thanks to longer integration times of each detection element in the line-scan camera as opposed to typical detectors utilized in the time-domain OCT spectrometers. As a result, Fourier-domain techniques are becoming prevalent in new generations of OCT instruments.
Detector 105 in a Fourier-domain OCT, then, typically includes a high-efficiency, high-resolution spectrometer with very high precision optics and mechanics. The focusing beam in detector 105 of a Fourier-domain OCT spectrometer typically needs to be aligned to the detector array on a line-scan camera at the micrometer level, resulting in imager 100 being very sensitive to any environmental change such as vibration and temperature. The OCT instruments are to be utilized in a clinical setting, where they are portable and where it will be impractical or very expensive to control environmental conditions. Under such conditions, maintaining alignment of the OCT imager can become a limiting problem. Therefore, there is need for constant alignment of the spectrometer to compensate for environmental changes as well as the effects of moving the OCT imager around the clinic.
Furthermore, Fourier-domain OCT imaging typically has a non-uniform noise background that varies along scan depth (i.e., along the A-scan) because of 1/f noise and other factors. The un-compensated image can have much stronger noise background at smaller depth locations. Furthermore, the noise background typically shows linear and other simple relationship with depth location and may vary from system to system because of localized factors specific to each spectrometer. In some systems, such noise levels may show a fixed noise background pattern that can be confused with the OCT image itself.
In addition, the basic Fourier-domain OCT imager is a DC signal system that may present other shortcomings. The signal purity may be affected by DC background levels resulting from the spectrum of light source 101. Additionally, the DC background may change over time due to environmental changes, which also makes improving the OCT image challenging.
Therefore, there is a need for OCT spectrometers and imaging systems that appropriately compensate for the above described factors.